Byron's moral reputation left a great deal to be desired. Why is this ironic when compared to the final 3 lines of "She Walks in Beauty"?

This is an interesting way to approach the poem "She Walks
in Beauty" because the issue seems to be raised in the poem itself.  In the first
stanza, Byron describes the beauty of a woman made of both darkness and light.  This
description applies not only to her physical looks, but to her soul as
well:

And all
that's best of dark and bright

Meet in her aspect and her
eyes

What has
caused the darkness, we can't tell.  Though, if the story about Byron's
cousin-in-mourning being the inspiration is to be believed, the answer is experience and
despair. If this is the answer, it does not jive with the final lines.  She may have
spent her days "in goodness," sure, but if she has lost someone she loves, it is hard to
believe that she would be either "at peace" or
"innocent."

As you say, the insistence on innocence and
goodness as the cause of the woman's beauty is ironic when compared to Byron's
reputation.  He was neither good nor innocent when it came to romantic entanglements,
though he could be described as both dark and light.  Perhaps what he sees in this woman
is what he wishes for himself: a return to innocence, while still retaining the dark
beauty of all his experience.